We plan to explore further the ability of post-thymic T cells to repopulate the various functionally distinct T-cell compartments in thymectomized, irradiated, bone marrow-supplemented syngeneic mice. Our work to date has shown that: (1)\there are at least two distinct post-thymic T-cell lineages (one for helper cells and one for cytotoxic cells); (2)\that only a small minority of post-thymic T cells (about 1%) are able to undergo expansion in a recipient host in the absence of thymic hormones or antigenic stimulus; and (3)\that these progenitor cells are able to undergo a numerical expansion of about 10,000-fold. This year's research will seek antigenic differences between the progenitor cells and the precursors they generate, will seek for qualitative differences between the precursors produced in these repopulated mice and their counterparts in normal mice, and will begin a study of the effect on the repopulation process mediated by purified lymphokines and specific antigen. (LB)